Xcom TFD added to the original: better graphics, new under sea environments, new weapons and new enemies.
The good enhancements:
Environment: The expansion of erie underwater environments , and the improved graphics in general were very welcome.
The strategic elements of building under-sea bases and under water fighter combat were both very cool. Although really only environmentally different not all that different otherwise.
New Options: Combat was slightly enhanced, more weapons were available, cooler looking armor, alien corpses, etc... (early on if a mission wasn't going well I would pack up an alien corpse for study in a operatives back pack... nostalgia...).
Weapons: some of the new weapons were very fun, sped up the pace of combat, & increased the tactical options of a squad (mainly the gauss technology tree).
The not so good (neutral to bad):
It bothered me that the smoke on land looked like muddy water.
Some things were ridiculously difficult... making the game pace move forward @ a snails pace (I generally would only allow one casualty before I would return to my last save).
The tech tree should have included @ least some of the advanced tech available for research in the first one… @ least for land use…
Over all this was a great game if overly difficult.